Monocrystalline silicon and GaAs are used in practical FETs. However, these materials are expensive, and the process for manufacturing the FETs is very complicated. In addition, the area of an FET is limited by the size of a silicon or gallium arsenide wafer. For example, when an active drive element used in a wide screen liquid crystal display element is manufactured, there are substantial restrictions on costs and area when a silicon or gallium arsenide wafer is used. Because of such restrictions, when an FET is used as a drive element in a liquid crystal display, a thin film transistor using amorphous silicon is presently used. However, in a thin film transistor using amorphous silicon, it becomes increasingly difficult to manufacture many elements uniformly on a plane surface at low cost as the display element area is increased. Thus, recently, it has been proposed that FETs be manufactured using an organic semiconductor material. Among organic semiconductor materials, those using a .pi.-conjugated polymer are especially useful because they are easily processed, which is a characteristic of a polymer material, and area can be easily increased (see Japanese Patent Publication 62-85224).
It is thought that a .pi.-conjugated polymer, whose chemical structure includes a conjugate double bond or triple bond, has a band structure comprising a valence band, a conduction band, and a forbidden band separating the valence and conduction bands which is formed by overlapping of .pi.-electron orbits. The forbidden band of the .pi.-conjugated polymer is mostly within a range of 1 to 4 Ev which varies with the material. Therefore, the .pi.-conjugated polymer itself has the electrical conductivity of an insulator or close to it. However, charge carriers are generated by removing electrons from the valence band (oxidation) chemically, electrochemically, physically, or the like or by implanting (referred to as doping hereinafter) electrons into the conduction band (reduction). As a result, it is possible to vary electrical conductivity over a large range, from insulating to metallic conduction, by controlling the amount of doping. The .pi.-conjugated polymer obtained by an oxidation reaction is p-type and by a reduction reaction is n-type. This result is similar to addition of dopant impurities to an inorganic semiconductor. Thus, it is possible to manufacture various semiconductor elements using the .pi.-conjugated polymer as a semiconductor material.
It is known that polyacetylene can be used as a .pi.-conjugated polymer in an FET (Journal of Applied Physics, Volume 54, page 3255, 1983). FIG. 15 is a sectional view showing a conventional FET using polyacetylene. In this figure, reference numeral 1 designates a glass substrate, reference numeral 2 designates an aluminum gate electrode, reference numeral 3 designates a polysiloxane insulating film, reference numeral 4 designates a polyacetylene semiconductor layer, and reference numerals 5 and 6 designate gold source and drain electrodes, respectively.
In operation of the polyacetylene FET, when a voltage is applied between the source electrode 5 and the drain electrode 6, a current flows through the polyacetylene film 4. When a voltage is applied to the gate electrode 2 disposed on the glass substrate 1 and separated from the polyacetylene film 4 by the insulating film 3, the electrical conductivity of the polyacetylene film 4 can be varied by the resulting electric field so that the current flow between the source and drain can be controlled. It is thought that the width of a depletion layer in the polyacetylene film 4 adjacent to the insulating film 3 varies with the voltage applied to the gate electrode 2 and thereby the area of a current channel varies. However, the amount of the current flow between the source and drain that can be varied by the gate voltage is small in this FET.
In other FETs, the .pi.-conjugated polymer is poly (N-methylpyrrole) (Chemistry Letters, page 863, 1986) and polythiophene (Applied Physics Letters, Volume 49, page 1210, 1986). FIG. 16 is a sectional view of an FET in which poly (N-methylpyrrole) or polythiophene is used as a semiconductor layer. In this figure, reference numeral 3 is a silicon oxide insulating film, reference numeral 4 is a poly (N-methylpyrrole) or polythiophene semiconductor layer, reference numerals 5 and 6 are gold source and drain electrodes, respectively, reference numeral 1 is a silicon substrate and gate electrode, and reference numeral 2 is a metal making ohmic contact to the silicon substrate 1. When poly (N-methylpyrrole) is used as the semiconductor layer, a current flowing between the source electrode 5 and the drain electrode 6 through the semiconductor layer 4 is only slightly controlled by the gate voltage, so there is no practical value in this FET.
On the other hand, when polythiophene is used as the semiconductor layer, the current flowing between the source electrode 5 and the drain electrode 6 through the semiconductor layer 4 can be modulated 100 to 1000-fold by the gate voltage. However, since polythiophene is formed by electrochemical polymerization in the prior art, it is very difficult to manufacture many uniform FETs at the same time.
Thus, the current between the source and drain which can be modulated by the gate voltage is too small in an FET in which polyacetylene or poly (N-methylpyrrole) is used as the semiconductor layer. In addition, although the current between the source and drain can be highly modulated by the gate voltage in an FET using polythiophene as the semiconductor layer and the FET is highly stable, since the FET is manufactured by forming a polythiophene film directly on the substrate by electrochemical polymerization, it is difficult to manufacture many uniform FETs on a large substrate at the same time, which is a problem.